We have determined hydration thermodynamics, _Ho298, _So298, and _Go298, for a family of monoalkyl ammonium ions, C1-C8 and 1-3 C1, using the electrospray ionization-equilibrium gas phase ion molecule reactions. The hydration thermodynamic values were calculated from equilibrium constants measured under conditions of ion source water partial pressures ranging between 0 and 100 mtorr, equivalent to 0-1% of total source pressure, and ion source temperatures ranging between 23 and 85oC. Our study clearly shows that 1) addition of the first water to a bare ion is apprecibly more exothermic than addition of the third; 2) addition of the first water to monoalkyl ammonium ions is about 2 kcal/mole more favorable for the case of monomethyl than for monobutyl and that there is relatively little change by further increases of alkyl chain length; 3) addition of the first water to monomethyl ammonium ions is about 3 kcal/mole more favorable for the case of trimethyl. Furthermore, addition of the first water to propyl ammonium is about 1.5 kcal/mole more favorable than the addition of the first water to trimethyl ammonium